Arboriculture & Urban Forestry 33(1): January 2007 65 depth present for seedlings and cuttings during the propaga- tion stage was maintained during nursery production to en- sure that when plants were transplanted to the field site, the root collar, or in the case of cuttings, the original sticking depth, was placed at the desired depth in the soil profile. Irrigation water for the nursery was injected with concen- trated sulfuric acid (Scholle Corp., Northlake, IL) to maintain water pH at 6.5 and with a 24N–3.5P–13.2K (24 to 8-16, 7.19% ammonium nitrate, 7.21% urea, and 9.60% nitrate; Scotts Corp.) water-soluble fertilizer to yield a concentration of 50 mg/L1 (50 ppm) N. Trees were staked and trained to a central leader during production. In May 2003, all five species were transplanted to a field Figure 1. Basal plate on a naturally growing American Beech, Fagus grandifolia Ehrhart, with the root flare at or above the soil surface. Grabosky, 2004; Arnold et al. 2005; Bryan et al. 2006). The objectives of this article were to investigate the responses of species from diverse families of trees and shrubs commonly cultivated in the southcentral United States to various plant- ing depths. MATERIALS AND METHODS During Spring and Summer 2002, five species of trees and shrubs from diverse families were propagated and grown in an outdoor container nursery at the Texas A&M University Horticultural Gardens in College Station, Texas, U.S. (lati- tude 30°6044N longitude 96°3122W) in 9.3 L (2.42 gal) (#3) black plastic containers (Lerio Corp., El Campo, TX) using a commercial pine bark-based substrate (3 milled pine bark:1 peat moss:1 coarse builders sand, by volume) amended with 6.8 kg/m3 (12 lb/yd3) of 18N–3P–8.3K con- trolled-release fertilizer (18 to 7-10; Scotts Corp., Marysville, OH), 3.4 kg/m3 (6 lb/yd3) of dolomite (Vulcan Materials Co., Tarrant, AL), 1.7 kg/m3 (3 lb/yd3) of gypsum (Standard Gyp- sum Corp., Fredericksburg, TX), and 0.68 kg/m3 (1.5 lb/yd3) of micromax micronutrients (Scotts Corp.). Fraxinus pennsylvanica and Platanus occidentalis trees were grown from seed, whereas Lagerstroemia indica × La- gerstroemia fauriei ‘Basham’s Party Pink’, Nerium oleander ‘Cranberry Cooler’, and Vitex agnus-castus ‘LeCompte’ were propagated from cuttings. Green ash and sycamore seeds were germinated in 38 cm (15.2 in) × 53 cm (21.2 in) × 10 cm (4 in) black plastic flats (Kadon Corp., Dayton, OH) contain- ing moistened substrate. Crapemyrtle, oleander, and vitex cuttings were approximately 15 cm (6 in) long. The basal 6.4 cm (2.56 in) were placed in the substrate during rooting on a mist bench. Propagules were transplanted to the 9.3 L (2.42 gal) (#3) plastic containers after rooting for cuttings (4 to 6 weeks after sticking) and when the first true leaves emerged on seedlings (2 to 3 weeks after sowing). The same planting site in College Station, Texas, by placing the root collars for seedlings, or original sticking depth for cuttings, 7.6 cm (3 in) below grade, at grade, or 7.6 cm (3 in) above grade. Field plots contained a Boonville Series, Boonville fine sandy loam, fine, montmorillic thermic ruptic-vertic albaqualfs (pH 9.1, bulk density 1.51 g/cm3, 61% sand, 11% clay, 28% silt) underlain at a 15.2 to 30.5 cm (6 to 12 in) depth with a hard clay pan. To ensure uniformity of disturbed soil volume within the planting holes, the holes were dug to accommodate the deepest planting depth possible (from the root collar to the bottom of the rootball plus 7.6 cm [3 in]) using a 45.7 cm (18.28 in) diameter auger mounted on a Dingo compact util- ity loader (The Toro Co., Bloomington, MN). This also en- sured that the clay pan was punctured to the same depth in all holes. Sides of the holes were scarred to avoid glazing and the backfill tamped firmly to achieve the desired planting depths. Final planting depths placed the root collars 7.2 cm (3 in) below grade, at grade, or 7.2 cm (3 in) above grade. The excised native soil was used as backfill during planting. No mulch was used, weeds were controlled by hand, and irrigation was provided using a commercial drip tape (T- Tape; T-Systems International Inc., San Diego, CA). Trees were irrigated daily during the first 4 weeks and thereafter when soil moisture reached −15 kPa (−15 centibars) as indi- cated by tensiometers (Model 2725 JetFill Tensiometers; Soil Moisture Equipment Corp., Santa Barbara, CA). The various taxa were arranged in a completely random factorial design of five taxa by three planting depths with six replicates of each taxa × planting depth combination. Tree height, trunk diameter, cross-sectional trunk area at 15 cm (6 in) above the soil surface, and shrub height, canopy spread, and cross-sectional trunk area were determined for three growing seasons. For trees/shrubs with multiple trunks, cross- sectional areas were calculated by totaling the calculated area for each stem using individual stem diameters. A canopy index (height × spread parallel to the row × spread perpen- dicular to the row) was also calculated as a volumetric esti- mate of the canopy size for the oleander, which remained more shrub-like in habit. Growth rates after transplant, ad- justed for initial size, were calculated for height, trunk diam- ©2007 International Society of Arboriculture
January 2007
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